Photoantioxidant activity

Al-Malaika, S. Ibrahim, A.Q. Rao, J. Scott, G. Mechanisms of antioxidant action photoantioxidant activity of polymer-bound hindered amines. II. bis acrylates. J. Appl. Polym. Sci. 1992, 44, 1287-1296. 

Table 15. Effect of Solvent Extraction on the Photoantioxidant Activity of Bound Antioxidants...

Identification of nitroxides in the oxidized hydroearbon substrate doped with HAS is considered as a proof of the photoantioxidant activity of HAS. The latter involves unique mechanistic features in polymer stabilization differing thus from the activity of conventional chain-braking antioxidants. The developed nitroxides are able to scavenge polymeric alkyls formed not only in the surface layers but also in areas of oxygen insufficiency in deeper polymer layers where they may be formed due to presence of residual oxygen. Participation of the polymeric alkyls in oxidative transformation into alkylpeioxyls POO" (2) is thus disabled. Resulting 0-alkylhydroxylamines >NOP are formed not only from >NH but also from various A-substituted HAS (>NX, X=alkyl, aryl, alkoxyl) and are considered as active reservoirs of HAS efficiency (6). 

Al-Malaika, S., Scott, G. and Wirjosentono, B. (1993) Mechanisms of antioxidant action Polymer-bound hindered amines by reactive processing, part III - Effect of reactive antioxidant structure. Polymer Degrad. Stab, 40, 233-238 Al-Malaika, S., Ibrahim, A.Q. and Al-Malaika, S. (1988) Mechanisms of antioxidant action Photoantioxidant activity of polymer-bound hindered amines I Bis maleate esters. Polymer Degrad. Stab., 22, 233-239 Al-Malaika, S. and Scott, G. (1995) US Patent 5382633. 

Table 10. Effect of Oxidative Processing on Photoantioxidant Activity of Dibutyi Thiophosphoryi Disuifide (DBDS) in PP ...

Figure 1.3 compares the photoantioxidant activity of a number of different UV stabilizers acting by different mechanisms [31]. The chain-breaking photoantioxidant (1076, IX) acts on the propagation cycle (Scheme 1.7). It does not...

Plastics have to be stabilized to withstand chemical and physical stresses during different phases of their lifetime. Stabilizers protecting plastics against particular degradation processes were developed and commercialized by various companies. According to their principal activity mechanisms, polymer stabilizers are conventionally classified as antioxidants, photoantioxidants, photostabilizers, heat stabilizers and... 

Attachment of B ansformation Products of Stabilizers. Up-to-date knowledge dealing with the chemistry of transformation products of phenolic [6, 15, 17, 20] and aromatic aminic [16, 43, 230] antioxidants and photoantioxidants based on hindered piperidines [10] indicates the possibility of attaching compounds having structures of quinone imine or quinone methide, or of radical species like cyclohexadienonyl, phenoxyl, aminyl or nitroxide to polymeric backbones. These reactions proceed mostly via reactivity of macroalkyl radicals derived fi-om stabilized polymers. Various compounds modelling this reactivity have been isolated [19, 230]. These results are of importance mainly for the explanation of mechanisms of antioxidant activity [6, 22, 24]. 

Thiol antioxidants can also be reacted with polyethylene and polypropylene (40). Figure 10 illustrates the superiority of MADA-B in PP compared with conventional thermal antioxidants when subjected to continuous hot water leaching. It is also an effective processing stabilizer and photoantioxidant. Table 15 shows that the latter effect is resistant to solvent extraction. The loss of activity after extraction corresponds to the amount of unbound MADA present. 

Photoactivation of FeDRC complexes produces iron carboxylates which are very powerful sensitizers of photo-oxidation due to their participation in redox reactions with hydroperoxides (37) (see Scheme 23) (in contrast, NiDRC complexes are not photoactivators since the librated fi ee metal ion has no photo-prooxidant activity). On the other hand, the observed photoinduction period at higher concentrations of FeDRC (see Fig. 5) is a direct consequence of its photoantioxidant behavior due to oxidation of the sulfur ligand to low molar mass sulfur acids... 

It has been shown in the preceding sections that raw polymers are highly susceptible to degradative oxidation. The success of plastic materials, which find apphca-tions in practically any aspect of life, relies heavily on the performance of polymer stabilizers, 70% of which are used for polyolefins. According to their principal protection activity, common polymer stabilizers are conventionally classified as antioxidants, photoantioxidants, photostabilizers, metal deactivators, antiozonants, and heat stabilizers for PVC. 

The aliphatic hindered amines (e.g. Ill) fall into the second class [30]. Neither they nor their active transformation products, the stable nitroxyls absorb UV light in the 300-350 nm region and the latter functions as a catalytic trap for alkyl and alkylperoxyl (see Scheme 1.6). The photoantioxidants are ultimately destroyed by attack of alkoxy 1 or hydroxyl radicals (formed by photolysis of hydroperoxides) on the heterocyclic ring. The hindered amines also synergize strongly with the UV absorbers. 

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